Behold Spartobranchus tenuis, a newly discovered creature that’s proving to be a crucial evolutionary missing link. The finding shows that biodiversity over half a billion years ago was more complex — and creepier — than previously thought. The phallus-shaped fossil was unearthed in Canada’s 505 million year-old Burgess Shale, and it’s a discovery that’s resetting the origin of these enteropneusts back to the Cambrian period.S. tenuis is a member of the acorn worms group — tiny and elusive creatures that frolic in fine sands and muddy waters. They’re part of the hemichordates, a group of marine animals closely related to today’s sea stars and sea urchins.Hemichordates were first discovered in the 19th century, but there’s been confusion about their origin and the relationship between its two main branches, enteropneusts and pterobranchs.S. tenuis now solves this mystery by connecting the two main hemichordate groups. Moreover, the fossil indicates that primitive enteropneusts developed a tubular structure, a characteristic that’s still featured by modern pterobranchs.

The tiny worms, which were found inside fossilized tubes, featured flexible bodies and were likely filter feeders (like other enteropneusts). The tubes were probably used as dwelling structures.The worms fed on small particles of matter filtered from seawater. The largest specimen was almost four inches long (10 cm). And fascinatingly, S. tenuis may have played an important role in moving carbon from the water column to the sediment in the early Burgess Shale environment.The discovery was made by Christopher Cameron of the University of Montreal’s Department of Biological Sciences and his colleagues. Their study now appears in Nature. More about the Burgess Shale here.